1. Field of the Invention
The present invention relates to a compression ratio controlling apparatus and method for spark-ignited gasoline internal combustion engine in which a variable compression ratio mechanism is equipped.
2. Description of the Related Art
A Japanese Patent Application First Publication No. 2002-21592 published on Jan. 23, 2002 which corresponds to a U.S. Pat. No. 6,505,582 issued on Jan. 14, 2003 exemplifies a previously proposed multiple-link type piston-crank mechanism. The previously proposed multiple-link type piston-crank mechanism is a mechanism in which a piston upper top dead center (TDC) position is changed by moving a part of the link mechanism. Such a kind of variable compression ratio mechanism as described above is a mechanism to vary a mechanical compression ratio, in other words, to vary a nominal compression ratio of the internal combustion engine. In general, during a partial load of the engine, the compression ratio is controlled to be at a high compression ratio to improve a thermal efficiency and is controlled to be at a low compression ratio to avoid an occurrence of an engine knock during a high load of the engine.